RADIO PULSARS ASP Conference Series, Vol.

**VOLUME***, 2003 Bailes, Nice €9 Thorsett

Improved Bounds on Violation of the Strong Equivalence Principle
Z. Arzoumanian Universities Space Research Association, Laboratory for High-Energy Astrophysics, NASA- GSFC, Greenbelt, MD 20771
Abstract. I describe a unique, 20-year-long timing program for the binary pulsar B0655+64, the stalwart control experiment for measurements of gravitational radiation damping in relativistic neutron-star binaries. Observed limits on evolution of the B0655+64 orbit provide new bounds on the existence of dipolar gravitational radiation, and hence on violation of the Strong Equivalence Principle.

1.

Introduction

PSR B0655+64, in a highly circular one-day orbit with a 0.8 M a white-dwarf companion, serves as a control experiment for measurements of orbital decay in the highly relativistic double-neutron-star binaries: General Relativity (GR) has predicted equally well the strong back-reaction to gravitational radiation for PSRs B1913+16 (Taylor 1993; see also Weisberg & Taylor, this volume) and B1534+12 (Stairs et al. 1998), and the absence of detectable orbital evolution for PSR B0655+64 over two decades. The long-term stability of the B0655+64 orbit sets unique bounds on departures from GR that give rise to dipolar gravitational radiation (Arzoumanian 1995, Goldman 1992), the existence of which would represent a violation of the Strong Equivalence Principle (SEP), one of the basic tenets of GR. G6rard & Wiaux (2002) examine the theoretical basis for dipolar gravitational radiation and suggest that bounds from binary pulsars may be competitive with future satellite experiments dedicated to probing SEP violation. A definitive analysis of the available data for PSR B0655+64, and implications for a variety of alternative theories of gravitation, will be published separately (Arzoumanian et al. 2003, in preparation). Following recent observations to extend our long-term monitoring program, I present here preliminary results on the orbital evolution of PSR B0655+64.
1.1.

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Binary Pulsars and the Strong Equivalence Principle

The Strong Equivalence Principle posits that the response of a body to an external gravitational field is independent of the gravitational self-energy of the body; it would be violated if, for example, two objects with different gravitational binding energies were observed to fall at different rates. SEP is satisfied by postulate within GR; as a consequence, the lowest allowed multipole order of gravitational radiation is the “electric” quadrupole. If SEP does not hold,
1

. in the Brans-Dicke theory. ~1 = 12. “quintessence. Other alternatives to GR are also constrained by limits on dipolar gravitational radiation (see.~ ..1 masyr-’. M G ml +m2. K D . At the level of the current constraint on the dipole term D .
+
s=-(-)
alnm alnG N
. 1998) or long-range forces that may be responsible for the acceleration. invokes the existence of a scalar gravitational field that couples to matter through a single dimensionless parameter.8 f 1. The Brans-Dicke theory becomes indistinguishable from GR as WBD becomes arbitrarily large..(
obs
=
(. Such fields would radiate predominantly at dipole order. solar-system experiments currently place a lower bound WBD 2 500. the fractional change in binding energy of each star with G.)Q+
(.g. using a new proper-motion measurement for B0655+64: . In a large class of metric theories of gravitation.rr/Pb. and a denote the effects of quadrupolar and dipolar gravitational radiation. a change in the gravitational constant with time.~ = 2(2 W B D ) . and To is the mass of the Sun in units of time. SEP can be tested with pulsar systems. Will 1981. Goldman 1992)
where n G 2. In GR. Because self-gravitational effects are important in neutron stars. G.
(4)
‘To correct for Galactic accelerations and the “Shklovskii” effect. the so-called “Brans-Dicke” scalar-tensor theory (Brans & Dicke 1961).the G contribution to i)b in Eq.)D+
@ ) G +
(a)
a
7
where the subscripts Q.2
Arzoumanian
however. ml and m2 are the pulsar and companion masses in solar units. 3 represent the “sensitivities” of the orbiting objects. I follow Damour & Taylor u = 6. and G describe the strength of quadrupolar and dipolar radiation and the effective gravitational constant in a given theory. K D G . The dimensionless parameters t q . and lends itself in principle to indirect detection through decay of a binary orbit at a rate inconsistent with the quadrupole-order prediction of GR. and the varying Doppler shift from a relative acceleration of the solar system and pulsar binary’.g.” Galdwell et al. The various contributions to observed changes in the orbital period pb of a binary system (neglecting tidal and mass-transfer effects) can be expressed as
). e. The quantities s 1 and s 2 in Eq. Kaspi et al. 1994). (1991). K D = 0. D. the radiation terms for a circular orbit are given by (Eardley 1975. The most thoroughly studied theory of gravitation that violates SEP. emission of dipolar gravitational radiation is allowed. WBD. 1 can be neglected (Arzoumanian 1995. Damour & Esposito-Farkse 1992): recent observations of distant type Ia supernovae suggesting that the expansion of the universe is accelerating have prompted renewed interest in theories involving scalar fields (e. and G = 1.

with larger values corresponding to “softer” equations of state.
1.l ~ y r . using the ELL^ dard procedures and analyzed with the TEMPO binary timing formula (Lange et al. Orbital phase residual and period measurements over time are shown in Figure 1. 2002).8 M a . ~ 3 strongly suppresses dipolar radiation from NS-NS binaries like B1913+16.~~
~
~~
_____
~~~~
~~~~
~
PSR B0655+64 and Dipolar Gravitational Radiation
3
where N is the total number of baryons.
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< 1.40 (Will & Zaglauer 1989). Results of X-ray burst oscillation modeling ( e g 7 Nath et al. and the companion was optically identified as a massive white dwarf by Kulkarni (1986). l).4 M a and 0. and the recent detection of atmospheric absorption lines from a neutron star (Cottam et al.2 below.a limit 7 times the GR prediction.0 x
obs
1 0 . The sensitivity of white dwarfs is negligible. and these data provide the interesting constraints on orbital evolution. 2002). because of the large difference in gravitational self-energy between the component stars. to obtain good coverage of all orbital phases at a single epoch. neutron stars are thought to have sensitivities in the range 0. Eq.
Observations and Results
The current data span 20 years. The constancy of p b confirms the result of the global timing solution: the available data bound changes in orbital period at the level IPblobs < 1.l (20). A least-squares fit of the entire dataset for rotational.~ 2 in) Eq.
(5)
. as well as the NRAO VLA (Thorsett 1991) and Jodrell Bank Love11 (Jones & Lyne 1988) telescopes. 140 Foot (Backus et al. If we assume pulsar and companion masses of 1. if such radiation exists.5 x (la). We carried out intensive observing campaigns.” Neutron star-white dwarf (NS-WD) binaries can therefore be expected to copiously emit dipolar radiation. and GBT telescopes. Pulse times-of-arrival (TOAs) were derived from observations following stansoftware package. Accounting for a small correction for relative acceleration (Eq.15 < s < 0. including observations made with the NRAO Green Bank 300 Foot (Taylor & Dewey 1988). and orbital parameters produces residuals consistent with statist ical fluctuations. Arzoumanian et al. gravitational binding energy takes on the role of an effective gravitational “charge. In scalar-tensor theories.
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2.2. astrometric. suggest that fairly stiff equations of state are appropriate. every few years with the 140 Foot and recently with the GBT. 1994b). 1982). we then have a 2 0 upper limit on orbit a1 evolution. It is worth noting that the difference (SI . 1982. 2001). pFR= -2 x corresponding to a decay timescale for the orbit of TQ 150 Gyr. 2 predicts a rate of orbital decay within GR. so that the celebrated agreement of the latter’s orbital decay rate with the GR prediction does not usefully constrain the existence of gravitational radiation at dipole order.
PSR B0655+64
PSR B0655+64 was discovered during a survey of the Northern sky made with the NRAO 300 Foot telescope (Damashek et al. I therefore adopt a fiducial value s = 0. Timing observations began soon thereafter.

. Box outlines depict ranges of dates over which phase and period measurements were made. From Eqs. 0.. we have in general
and for the Brans-Dicke theory specifically. The three curves in each set represent the ( m l . 3.7). WBD
2 320 ( ~ / 0 . and 5.. a-
0
E
.
I I
..35. 2. pulsar surveys continue to discover NS-WD systems similar to B0655+64 but with shorter orbital periods and millisecond pulse periods.. Eqs. Their higher rates of gravitational energy release coupled with higher timing precision suggest that these systems will surpass B0655+64 as laboratories for
. ~
3 . Damour & Taylor (1992) show that. Also. 2..9). measurement uncertainty for & scales with data span T as T-5/2. Horizontal lines depict the current la and 2a limits on orbital decay in the PSR B0655+64 system.. 2-3.Arzo umanian
\
Q
4. 0. The curving dashed and dotted lines are the expected orbital period changes due to emission of gravitational radiation through quadrupole order. for constant data quality. data quality typically improves with time through improved instrumentation. 2 )(20). 0. The resulting constraints on WBD are summarized in Fig. for NS sensitivities from soft and stiff equations of state respectively.40.. so that total system mass increases to the upper left.30. Constraints on WBD then lie at the intersections of the dashed or dotted curves with the measured upper limits on &. (1. Discussion
Prospects for improving bounds on SEP violation and the existence of dipolar gravitational radiation are good.
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Date
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Figure 1.8). Orbital evolution of PSR B0655+64.. moreover. and (1. m2) pairs (1.